Different Techniques in Qualitative and Quantitative Elemental Analysis

Elemental analysis methods are used to determine the elemental composition of a substance and the concentration of each element present. However, there are two different subsets: one for determining what elements are present, and another for measuring the quantities of elemental constituents. These are known as qualitative and quantitative elemental analyses, respectively. In this article, we look at these different elemental analysis sub-classes and look at some of the main techniques that fall into both categories.

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Like many analyses, quantitative analysis provides a numerical output, whereas qualitative analysis provides a non-numerical output. It is the same for quantitative and qualitative elemental analyses - quantitative analysis produces information about the mass of each element in a sample (which enables the concentration of a substance to be determined), and a qualitative elemental analysis provides information on the different types of elements present. In many circumstances, qualitative and quantitative analyses are used in conjunction with each other to build up a complete picture of the sample being analyzed.

In some instances, the choice of technique can also be influenced by a few different factors, namely, how much information is already known about the sample, what information needs to be determined and whether the analysis is for a surface, thin layers, or bulk materials.

Below, we're going to look at some of the quantitative methods, qualitative techniques, and ways in which both types of analyses can be performed. It is not an exhaustive list (as there are many out there), but it highlights some of the most common methods.

Qualitative Analysis Techniques

Various methods can be employed to detect if a particular element is present within a sample without determining its concentration. Many of these methods are specific to analyzing one, or a select few, element(s).

They include the sodium fusion test (determines the presence of halogens and sulphur atoms in an organic compound), the Schöniger oxidation reaction (used to identify any elemental chlorine, sulfur or nitrogen in a sample), the acid test (to examine if gold is present in a material), and the Kastle-Meyer test (examines the sample for the presence of blood).

Mass spectrometry (MS) and inductively-coupled mass spectrometry (ICP-MS) are two techniques that could technically fall under a method that is both qualitative and quantitative. However, its primary function is to separate elements based on their mass-to-charge ratio, which enables the elements present in the sample to be determined. This process alone does not yield quantitative data automatically. The concentrations of each element can be determined, but the results need to be calibrated against known standards.

As scientific instrumentation has advanced, most of the techniques that were once solely qualitative can now be used to perform both types of analyses. One example of this is microscopy, as there are now many methods for qualitatively analyzing the sample, in addition to taking an image of it.

Quantitative Analysis Techniques

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There are also methods that can be used to determine the concentration of a substance. But unlike the qualitative techniques, these are methods that can be used on a wide range of samples.

Gravimetric analysis is one such method. It generates a precipitate through chemical reactions and uses the known mass of a formed precipitate to back out the amount of the original material after it has been weighed and calculated using the known molecular formulae.

The other primary method is titration. There are various ways in which these can be done, but is primarily used to measure metal ions, as they form many better-colored complexes that can easily signal when the reaction has finished. The amount of chemical solution titrated, and the known chemical formulae can be used to determine the concentration of ions in a sample.

Again, much like qualitative analyses, many methods have now been developed that can efficiently perform both types of analyses and this means that most of the standalone qualitative and quantitative techniques are methods which are shown in a lab via wet chemical methods rather than on an analytical instrument.

Techniques that Perform Both

There are also many versatile techniques that can be used to determine the elements that are present in a sample, as well as how many of each element are in the sample.

X-ray photoelectron spectroscopy (XPS) is a technique that measures the emissions of electrons from a sample (after being irradiated with X-rays), and the analyzer can deduce qualitative and quantitative information about the chemical make up of the sample. It is a technique that is best suited for surface analysis, but it can also be used for bulk materials and thin layer materials.

There is also atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES) and ionized coupled plasma atomic emission spectroscopy (ICP-AES), which can be used to perform both analyses. AAS records the wavelengths of light that are absorbed by the elements in the sample, whereas AES and ICP-AES measure the wavelengths emitted by the sample after it has been excited.

In all these cases, the wavelengths absorbed and emitted are specific for each element, and this enables the types of element to be deduced. The intensity of the wavelengths emitted, or the degree by which they are absorbed allows the concentration of elements in the sample to be deduced. ICP-AES is an ideal technique for a bulk analysis as it can quantify both major and minor elemental constituents.

Another technique is X-ray fluorescence (XRF)—and energy dispersive XRF (EDXRF)—which causes the sample to emit secondary fluorescent X-rays after the sample has been excited with high energy X-rays. The emitted X-rays are characteristic for each element, and this enables the elements in the sample to be determined. The intensity of the X-rays gives information on the concentration of the elements in the sample. XRF techniques are another useful technique for quantifying the major and minor elemental constituents in a bulk sample.

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